Optical fiber sensors have certain advantages including being unaffected by electromagnetic interference, lightweight, small size, high sensitivity, large bandwidth, and ease in signal light transmission. In recent years, the use of optical fiber sensors for monitoring pressure, strain, displacement, temperature and humidity has become increasingly common.
Three techniques have been applied in the polymer optical fiber (POF) sensor field: interference technology, fiber Bragg diffraction grating (FBG) technology and optical time-domain reflectometry (OTDR). All three techniques are highly sensitive, but they require complex and expensive measurement equipment. Measurements are very sensitive to temperature changes and therefore require complicated compensation techniques. Furthermore, these sensors have a limited strain range for repeated large deformation.
There have been experimental investigations into crack detection and vertical deflection monitoring in concrete beams using POF sensors in which a segment of the cross-sectional profile was deliberately removed in order to enhance the sensing performance.
An optical displacement sensor has been developed based on bending multilooped POFs that has multistructural imperfections on the outer side of its core. The power loss was found to be particularly sensitive to the depth of the increased imperfection.
A macrobending POF has been used for weight measurement. The macrobending POF is used with an inexpensive and simple sensor having intensity modulation. When a plastic optical fiber is fully and rigidly adhered on a metallic cantilever beam, it is sufficiently sensitive for the measurement of force in the form of weight.
A curvature gauge sensor has been modeled by geometric optics. A fraction of power refracted at each of many planes of the serrated sensitive zone was calculated separately. The model enabled visualization of fundamentals involved in the operation of this fiber-optic sensor so that a better qualitative understanding could be gained.
WO 94/29671 discloses a curvature or bending and displacement sensor composed of an initially straight fiber optic or light wave guide. The sensor is for attachment to a member or members being curved or displaced. Curvature of the fiber resulted in macro-bending loss sometimes in conjunction with a superimposed curvature in a plane other than that of the curvature to be measured. This macro-bending loss is detected. The loss of light detection was used to produce indication of curvature or displacement.
U.S. Pat. No. 5,900,556 discloses a helical optical fiber sensor based on macrobending losses. The sensor is embedded in concrete and is used to monitor the deformation (strain) of concrete due to compression, tension, and bending. The material of the helical optical fiber was silicone rubber. The range of the deformation and the sensitivity of the sensor were not disclosed.